1. Field of the Invention
This invention relates to therapeutic treatment for the removal of intravascular blockage due to atherosclerotic plaque and intravascular blood clots and, more particularly, to a method and apparatus for the removal of atherosclerotic plaque and blood clots in blood vessels by the use of ultrasonic energy.
2. Description of the Related Art
For proper health, the arteries must have sufficient elasticity to expand with each heartbeat and to withstand the high pressures of arterial blood flow. When cholesterol, fibrous material, and other substances coat the inner surface of arteries, there can be a severe loss in blood vessel elasticity. This condition is commonly referred to as atherosclerosis. In addition to cholesterol and fibrous materials, calcium deposits may accumulate that can further reduce the elasticity of the blood vessel and can cause the accumulated deposits to become hard.
The deposits are most frequently associated with the blood vessels supplying the heart, but the deposits may occur elsewhere in the body. These deposits, which are generally referred to as plaque, accumulate at one location and drastically narrow the bore of the artery, thus restricting and even totally blocking the flow of blood. Plaque has a thick, viscous consistency. The site of accumulated deposits that constrict the blood vessel is referred to as a stenosis while a total blockage is referred to as an occlusion. Similarly, a blood clot is often associated with the blood vessels supplying the heart, but may occur elsewhere in the body. An intravascular blood clot is generally referred to as a thrombosis.
Many different techniques and devices have been used in the treatment of atherosclerosis and thromboses. Each of the techniques suffers from deficiencies that make an alternative procedure desirable. These techniques include arterial bypass surgery, endarterectomy, balloon angioplasty, applications of laser technology, mechanical and electrical drills, and other surgical techniques and tools for the removal of the deposits.
Arterial bypass surgery involves the bypass of the narrowed or occluded arterial segment by using a synthetic conduit or an arterial or venous graft to carry blood past the arterial obstruction. Arterial bypass surgery requires surgery, with the attendant risks of surgery and general anesthesia, problems of wound healing and infection, post-operative complications, and the problem of post-operative graft closure.
Endarterectomy also requires direct surgery, but further requires that the blocked blood vessel itself be opened and the blockage removed. That is, the blood vessel is exposed during surgery, the vessel is slit open along the blocked portion, and the blockage is manually removed by the physician. The slit must then be sealed up so that blood may once again flow through the vessel. Thus, this technique is in many ways more invasive than bypass surgery and includes all of its potential shortcomings.
Balloon angioplasty is a recent development that involves the insertion of a catheter having a circumferential balloon attached at its distal end. A thin, flexible guide wire is first inserted in the blood vessel and is advanced to the site of a stenosis. A catheter is then slipped over the guide wire and is advanced in the blood vessel along the length of the wire. Once the catheter is at the stenosis, the balloon is inflated and thereby dilates the stenotic arterial segments. This stretches the artery and may obviate the need for arterial bypass operations. The limitations of balloon angioplasty include arterial dissection, bleeding, and re-occlusion. Furthermore, most total stenoses or occlusions and totally calcified blockages cannot be treated by balloon angioplasty techniques, as balloon angioplasty does not destroy or pulverize arterial plaque or clots. In fact, very few non-surgical techniques are capable of opening up a totally calcified blockage. A "hot-tip" catheter, for example, can be used for most fibrous blockages but cannot be used for totally calcified blockages. A hot-tip catheter also has an increased risk of perforation.
Efforts are also under way to apply laser technology to dissolve or vaporize plaque. There currently are no laser techniques in widespread use. Laser techniques, however, have a high damage potential related to perforation of the blood vessel. In such a case, direct visual observation can be extremely important in avoiding damage to healthy tissue.
Drill techniques typically employ a stiff, threaded probe that rotates and thereby drills or reams out the plaque. These techniques also present a high damage potential, primarily due to perforation, and therefore direct visual observation of the site may be necessary. For direct visual observation, it may be necessary to use angioscopy to insert a fiber optic probe into the blood vessel, further complicating the treatment process. Angioscopy involves the insertion of an angioscope, or fiber optic probe, into the blood vessel for visual inspection of the treatment site. Alternatively, direct surgical techniques would be necessary. Thus, there is a need for a device that will dissolve or pulverize atherosclerotic plaque and intravascular obstructions without these dangerous and cumbersome complications.
The use of ultrasonic energy for the destruction of plaque has been suggested, but prior techniques and devices still suffer from deficiencies and there have been no successful applications of the technology in treatment. For example, U.S. Pat. No. 3,565.062 to Kuris utilizes a catheter inserted into a blood vessel and requires a procedure in which the blocked artery is surgically exposed. Thus, many of the potential problems of arterial bypass surgery are present. In addition, a hollow vibrating probe is provided in conjunction with a catheter having slots for catching strips of plaque to be shaved off. The apparatus and accompanying procedure are quite involved and time-consuming, and require much manipulation.
The device of U.S. Pat. No. 3,526,219 to Balamuth utilizes ultrasonic energy but is used for the removal of tumors, warts, moles, skin cancer, and others from the surface of skin tissue. The device has no application to the interior of blood vessels. The device of U.S. Pat. No. 3,433,226 to Boyd relates to a catheter that vibrates at an ultrasonic resonant frequency to destroy plaque. Because the catheter itself vibrates adjacent to the blood vessel wall, and is advanced in the blood vessel for destruction of the stenosis, there is a danger of perforation of the vessel wall during application of ultrasonic energy. Thus, these previous ultrasonic methods of treating plaque still include many undesirable complications and dangers.